INTRODUCTION
Chassis Plans' XPT single board computer, model number S6090, uses a unique cooling solution designed to provide reliable system operation at elevated ambient temperatures. Many of today's SBC cooling solutions leave damaging processor-generated heat in the chassis and rely on the system fan or fans for removal. Also, heat generated by other SBC components is often simply radiated into the system enclosure. The XPT's cooling solution is designed to prevent a premature system failure or a temporary shutdown situation by cooling the Intel^® Xeon processors, and other SBC components, while channeling the heated air out of the chassis. The net result is an effective SBC cooling solution that maximizes system up time leading to more efficient operation with faster return on investment.

BACKGROUND INFORMATION
It's an engineering fact of life that as silicon-based microprocessors increase their operating speeds, the resultant power consumption also increases. Compounding this situation is the fact that as the processor's speed increases, the allowable processor case temperature (Tcase) decreases. With today's Xeon processors, cooling systems must be designed around a maximum power dissipation of 101 Watts with a maximum Tcase rating of 73° Celsius. In the XPT the dual Xeon processors can theoretically dissipate up to 202W. The power needed to drive these processors demands voltage regulation circuitry, i.e. VRMs, that also generates heat. Other SBC components such as the chipsets, bridges, SCSI, video and Ethernet controllers generate additional heat, so the design engineer must have a plan to effectively manage and control SBC board temperature in order to maximize system reliability and value. The XPT cooling solution cools all of the SBC components, not just the processors.

DESIGN REQUIREMENTS
The prime engineering directive forChassis Plans' XPT cooling solution was to design a system that allowed the SBC to be specified and validated for operation at a maximum ambient temperature of 45° C. This validation was to be performed under a variety of high temperature and system loading conditions, and the cooling solution needed to have a safety margin of several degrees. These design requirements pointed to a solution that cools all SBC components while aiding in the removal of heat from the chassis enclosure. Figure 1 shows the XPT's major heat-producing components.

The XPT cooling solution effectively maintains proper thermal operating conditions for these components in the smallest package possible in order to maximize SBC reliability in a wide variety of applications.

Figure 1

OVERVIEW
Figure 2 shows two views of the XPT, i.e. one with the cooling solution installed and one with it removed, in order to show the key components of the assembly.

Figure 2

The voltage that drives the dual speed fan is controlled by a thermal switch/sensor located on the XPT's I/O board. SBC board temperatures are much higher than external ambient temperatures. The switch delivers maximum voltage to the fan when the sensor portion of the device reads a board temperature over 48° C. This causes the fan to operate at a maximum speed of 50 CFM. Under normal operating conditions, the fan operates at approximately 37 CFM. Running the fan at full speed only when necessary maximizes fan reliability and MTBF. An air deflector is used to direct a small amount of cooling air over the XPT's VRM and I/O board components. Board stiffeners are used to provide support and minimize board flexure.

MECHANICALS
Figure 3 illustrates how the XPT, with the cooling solution installed, fits into a typical PICMG^® backplane.

Figure 3

The XPT's cooling solution accommodates the placement of half-size PCI cards approximately 2.3" (58.4mm) away from the SBC. Full-length cards must be placed approximately 3.2" (81.3mm) away from the XPT.

AIR FLOW
Cool air is drawn through the front of a typical enclosure chassis and into the XPT's shroud by the dual speed fan. The cool air warms as it passes over various SBC components and is exhausted through the I/O bracket as shown in Figure 4.

Figure 4

The shape of the XPT's shroud is designed to maximize the air flow over critical SBC components. An air deflector is part of the shroud and directs low pressure cooling air between the two boards in the XPT single board computer. Figure 5 illustrates the direction of the air movement through the XPT's shroud.

Figure 5

The passive heat sinks mounted on top of the Xeon processors have a height differential of 10mm in order to ensure that both processors are adequately cooled. The difference in the heights of the passive heat sinks can be seen in Figure 2. Some cooling air is directed over the VRM circuits in order to maintain optimum performance during peak power demands.

SUMMARY
The cooling solution on the XPT single board computer maximizes system reliability by effectively cooling the SBC while aiding in the removal of heat from the enclosure. In the future, microprocessors will continue to increase in speed and power consumption, leading to increased heat dissipation. Newer manufacturing processes have the potential of changing this dynamic; however, these improvements will not change the basic fact that faster processors will always run hotter. Chassis Plans' engineers will continue to design innovative cooling solutions for the processors and other SBC board components which are required for demanding applied computing environments.

Intel and Xeon are trademarks of Intel Corporation.
All other product names are trademarks of their respective owners.

Copyright ® 2003 by Chassis Plans, LLC.
All rights reserved.